You probably figured this out with playing, but I'll answer anyways...
1) Five stages (but more to come)!
2) There's the "Stats" page for everything. It's not like there's a lot of numbers...the main things are like weight, capacity, calories, eating speed and digestion rate.
3) There will be an item catalog, of sorts! In the future items you haven't obtained will show in the inventory, just grayed out.
It is quite the cursed berry...
https://bulbapedia.bulbagarden.net/wiki/Pomeg_Berry#Trivia